MULTIDISCIPLINARY CHARACTERISATION OF THE STUDY SITES

The investigation area is located in Val di Rabbi in the south Alpine belt in northern Italy (Fig. 2). This area was chosen due to (i) its situation between a rather warm Insubrien and a cold Alpine climate; (ii) the general suitability of the sites; (iii) the existing and well-established network including the good collaboration with the local authorities; and (iv) the comprehensive database about soils and GIS data. 

Specifically, ten sites along an altitudinal gradient ranging from 1200 up to 2400 m a.s.l. have been investigated. Five sites are positioned on north-facing slopes (N1-5) and other five on south-facing (S6-10) slopes (Fig. 3). To facilitate the comparisons between soils collected on N and S slopes, the altitudes of the sites from which they have been taken are as similar as possible. Along the studied altitudinal gradient, the land use varies from different types of forests (N1-4; S6-9) at lower elevations to alpine grasslands (N5 and S10) at higher elevations. We have selected old-growth forest sites and natural grasslands in order to minimise the influence of human activities and the grazing by livestock. Overall, all the sites are on paragneiss or morainic material consisting of paragneiss (Egli et al., 2006), and the soils are classified as Cambisols to Umbrisols or Podzols. 

Fig. 2. Localisation of Val di Rabbi (Trentino, Italy) and the different study sites N1-5 (N: north-facing) and S6-10 (S: south-facing). This figure was taken from Egli et al. (2006)* - *Egli et al. (2006) Catena 67:155-174

Fig. 3. Overview of the study sites along an altitudinal gradient at north (N1-N5) vs south (S6-S10) exposure, representing microclimosequences, located in Val di Rabbi (Trentino, Italy). The fotos (J. Ascher) were assembled by Tommaso Bardelli (Master Thesis, Bardelli 2014) on the modified scheme of Egli et al. (2006).

Overall, at each climosequence site the content and quality of organic matter (using a chemical and physical fractionation; ∂13C, ∂15N and lignin component ratios), humus forms, activity and composition of faunal (microannelids, Enchytraeidae) and microbial communities (bacteria, archaea, fungi) are being assessed. The close relation between humus forms and the soil biota is based on the fact that humus forms result mainly from the activity of soil organisms and at the same time act as a habitat for them. By using the knowledge about soil organic matter (SOM) evolution that finally gives rise to a limited number of humus forms, both the upscaling of coarse woody debris (CWD) degradation processes (using GIS) and the integration into the soil at regional and landscape level will be made possible. Three submodels are being created (a small-scale site characteristic model, a CWD and a regional humus model) that are the basis for the final model.

 

More specifically, in August 2012 the first general screening of each climosequence site was performed in function of the slope exposure and soil depth. Briefly, three plots (5x5 m) at 50 m from each other were set-up in each of the 10 study sites and five soil sub-samples were randomly taken from each plot in 5 cm depth intervals (0-5; 5-10 and 10-15 cm), using a corer device (ø 5 cm; 5 cm correspond to 100 cm3; Fig. 4). Soil samples were placed in polyethylene bags and transported on ice to the laboratory for chemical and molecular characterisation. In addition, the determination of humus forms and microannelid fauna was also assessed for each of the ten sites by using the same sampling device (Fig. 4). The performed sampling strategy provides the basis for accurate mesofauna-microbiota interaction studies, descriptive for humus forms and overall ecosystem functioning.

Fig. 4. Overview of the soil sampling campaign (August 2012) using a special corer device (U. Graefe; 0-15 cm) allowing for classification of humus forms and accurate assessment of microbiota-mesofauna interactions. Photos taken by J. Ascher.

Furthermore, a characterisation of CWD was also carried out in the different sites in June and August 2013, according to Heilman-Claussen and Christensen (2003) Biodiversity and Conservation 12: 953-973. CWD offers a variety of essential functions in the forest ecosystem, as it provides seed germination sites, serves as reservoirs during droughts, and provides habitats for many forest animals and microbes (Fig. 5). Moreover, CWD also plays an integral role in the material flow, energy flow and nutrient cycling, as it can release carbon, nitrogen, phosphorus and other nutrients gradually and tardily by its decomposition.

Fig. 5. Overview of deadwood logs as reservoirs for fungal primary decomposer communities. Photos taken by J. Ascher.

CWD can be classified into five subcategories: (I) hard wood, penetrable with a knife to only a few mm, bark and twigs (diameter <1 cm) intact; (II) rather hard wood, penetrable with a knife to less than 1 cm, bark and twigs begin to shed away, branches (diameter 1–4 cm) intact; (III) distinctly softened wood, penetrable with a knife to approximately 1–4 cm, bark and branches partially lost, original log circumference intact; (IV) considerably decayed wood, penetrable with a knife to approximately 5–10 cm, bark lost in most places, original log circumference begins to disintegrate; (V) wood that disintegrates either to a very soft crumbly texture or is flaky and fragile, penetrable with a knife to more than 10 cm, original log circumference barely recognizable or not discernable. 

 

Taking into account the previous CWD classification, samples from the five different decay classes have been collected along the climosequence sites (Fig. 6). All the samples were placed in a coolbox until they were taken to the laboratory (Fig.7a,b), and they were treated using a cutting-mill for chemical and molecular characterisation. (Fig. 12b, “mesocosm experiment”)

Fig. 6. Overview of the sampling procedure of CWD in the field experimental areas. The photo on the left side was taken by Lucie Darbellay; the photos located in the middle by J. Ascher; and the photo on the right side by Dylan Tatti.

Fig. 7b. Overview of the five different decay classes. Photos taken by J. Ascher.

 

Fig. 7a. Overview of the five different decay classes of Picea abies (from the left to the right side: classes 1-5). Photos taken by J. Ascher.

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